of Epitaxial AlN/GaN/GaInN Photocathode Heterostructures
D.J. Leopolda,b, J. Buckleya, W. R. Binnsa, P. Hinka, and M.H. Israela
Department of Physics and McDonnell Center for the Space Sciences
Washington University, St. Louis, MO
A need for extending the sensitivity of photon detectors to the blue and UV wavebands comes from the fact that both Cherenkov light and scintillation light typically have an emission spectrum that is peaked at short wavelengths. Photocathode layers consisting of wide-band-gap AlN/GaN/GaInN heterostructures are being compositionally tailored on an atomic scale during the epitaxial crystal growth process to control photoelectron absorption, diffusion, and transport to the negative electron affinity cathode surface. These nitride alloy heterostructure layers are being grown directly on transparent single crystal sapphire substrates in ultra-high vacuum by molecular beam epitaxy. This technology is expected to significantly enhance UV/blue radiation detection sensitivity in single photon counting imaging and non-imaging devices.
Direct space-based imaging of rapid UV/blue astrophysical transients will greatly benefit from photon counting detectors with high quantum efficiency. Also X-ray, Gamma-ray, and cosmic-ray experiments that employ scintillation light detectors or Cherenkov detectors would benefit greatly from photomultipliers with higher quantum efficiencies. In this case the need for increasing the sensitivity of photon detectors in the blue and UV wavebands comes from the fact that both Cherenkov light and scintillation light have an emission spectrum that is peaked at short wavelengths. Photomultiplier tubes in use today for high-energy particle detection applications have a significant spectral mismatch with typical sources. This point is demonstrated in Fig. 1 , where we have displayed a typical Cherenkov light spectrum as well as the emission from a plastic scintillator together with the detection sensitivity curve of a bialkali photomultiplier tube and a tube incorporating a state-of-the-art GaAsP photocathode. The short wavelength response of the GaAsP tube shown in Fig. 1 has been enhanced through the use of a wavelength shifter coating on the window .
Photomultiplier tubes are high-gain optical detectors capable of photon detection over a wide spectral range. The heart of such a device is the photocathode. By the photoelectric effect, a photon incident on the photocathode ejects an electron which then is accelerated by an electrical potential to produce a number of secondary electrons either through a cascade of interactions in a dynode chain or through a direct interaction with a photodiode. These secondary electrons constitute the measured signal. The quantum efficiency of such a device is defined in terms of an optical to electrical conversion percentage, determined by the number density of initial photoelectrons produced per incident photon flux.
Our research on high quantum efficiency photocathodes involves the design and fabrication of precisely tailored heteroepitaxial semiconductor structures that have peak sensitivity in the UV/blue spectral range. After considering the optoelecronic and structural properties of different wide-band-gap semiconductor alloy materials we have determined AlN/GaN/GaInN to be the most suitable candidate. The band gap of this system can be tailored over an energy range from 1.9 to 6.2 eV and epitaxial thin film layers can be grown directly on optically transparent sapphire substrates . GaAlN/GaInN has already been recognized as a leading material in the fabrication of wide-band-gap laser diode devices, making it a logical choice for heteroepitaxial photocathode development. The AlN/GaN/GaInN heterostructures discussed in this work have been fabricated in ultra-high vacuum by molecular beam epitaxy (MBE). The use of MBE for crystal growth makes it possible to control film composition on an atomic scale and to fabricate abrupt heteroepitaxial interfaces. The ultra-high vacuum conditions and cryogenically cooled chamber walls allow for very quick on/off switching of atomic and molecular beams through the use of shutters in front of each thermal or electron beam source. A reflection high energy electron diffraction (RHEED) system mounted inside the vacuum chamber allows the surface crystal quality to be monitored and individual atomic layers to be counted during growth as they are added to the surface one at a time by examining surface reconstruction diffraction patterns. This feedback provides the absolute finest control of the heteroepitaxial crystal growth process, thereby making possible precise fabrication of semiconductor layered structures for use in high performance devices
All AlN/GaN/GaInN heterostructures
used in these studies are grown on single-crystal sapphire substrates.
The mechanical strength and UV/visible optical transparency properties
of sapphire make it an excellent choice as a window material for photocathode
structures. In order to increase the quantum efficiency of AlN/GaN/GaInN
photocathodes a couple of key design features are incorporated in our
Examples of this are shown in Fig.
2 , where conduction and valence band edge energy spatial profiles
are displayed for two possible photocathode designs. This diagram is basically
a plot of band gap versus depth into the photocathode structure.
For clarity the individual layer thicknesses shown in Fig.
2 are not drawn to scale but instead should be regarded as a rough
schematic to illustrate the main design features.
An AlN optical antireflection layer inserted between the sapphire
and the GaInN photocathode region serves as a wide-band-gap barrier to
prevent electronic back diffusion into the substrate interfacial region
where defect densities are expected to be higher and nonradiative recombination
of photoexcited electrons larger.
Inserting this wide band gap AlN buffer layer in the structure
ensures that photoexcited electrons do not diffuse back toward the sapphire
substrate interface, but instead are reflected toward the photocathode
At the present time AlN/GaInN photocathodes have been designed and are being fabricated on up to 2-inch diameter sapphire substrates by MBE. The structural, optical, and electronic properties of these heterostructures are being evaluated. X-ray diffraction and TEM lattice image studies show good registry of epitaxial GaN and GaInN layers with the c-plane-oriented, single-crystal sapphire substrates. Optical absorption measurements of GaN and GaInN confirm the expected band gap shift with increasing indium concentration. Also, a photoelectric emission measurement stage is being designed for use in the ultra-high-vacuum MBE system. This stage will be used to measure the quantum efficiency and spectral responsivity of as-grown AlN/GaInN photocathode heterostructures without breaking vacuum. Overall this new nitride-based photocathode technology is expected to have a significant impact on Cherenkov and scintillation radiation detection where emission is peaked in the UV/blue spectral range, and on direct space-based imaging of rapid UV/blue astrophysical transients.
This research is supported by NASA Grant # NAG5-8536 under the Explorer Technology Program.